This invention relates to compositions of matter classified in the art of chemistry as low volatile organic compound (xe2x80x9cVOCxe2x80x9d) liquid coating compositions, more specifically to such low VOC liquid coating compositions based on polymers derived from polymerization of acrylic monomers, to processes for their preparation and use, to coatings derived therefrom and to objects coated on at least one surface thereof with such coatings.
Increasingly stringent emissions regulations have increased the need on the part of makers of acrylic lacquers and paints for formulations which provide the desirable characteristics associated with such lacquers and paints but with reduced VOC content. One way to achieve such formulations is to increase the solids content of the formulations while maintaining the viscosity of the paint or lacquer within the range required for application, usually spraying, and thereby reduce the amount of VOCs emitted when a given amount of surface area is coated.
Spray application requires low viscosity. Viscosity is directly related to polydispersity (as this term is further defined hereinafter) and concentration of the resin(s) in the paint or lacquer formulation. In order to maintain low viscosity at high resin solids content, low polydispersity of the resin(s) is necessary.
U.S. Pat. No. 4,777,230 discloses that t-amyl peroxides and peroxides of similar structure provide polydispersity as low as 2 in solution polymerization of acrylic resins.
Organometallic complexes and thio compounds have been shown to provide controlled polymerization reactions, see Macromolecules, 1995, 28, 1721-3, Kato, et al, J. Am. Chem. Soc., 1995, 117, 5614-5, Wang, et al and WO 98/01478. These procedures require the removal of the residual organometallic complex or thio compound from the product to produce a practically useful material.
Controlled radical polymerizations using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy radical) and analogous compounds as well as their alkoxyamine analogs have been reported to produce desirable properties for vinyl polymerizations. See, for example, U.S. Pat. No. 5,322,912, Macromolecules, 1993, 26, 2987-8, Georges, et al, U.S. Pat. Nos. 4,581,429, 5,627,248 and WO 98/13392. None of these references indicates that this type of polymerization reaction will provide acrylic polymers with the desirable properties found for the products produced by the processes of the present invention. In fact, examination of the data shown, for example, in examples 29 through 31 and comparative example K, shows very little difference for polymerization of the acrylic monomer employed by either the particular controlled radical xe2x80x9ciniterxe2x80x9d used or the organic peroxide.
In Chapter 14 of xe2x80x9cControlled Radical Polymerizationxe2x80x9d, Matyjaszewski, K., Ed. ACS Publication Division and Oxford Univ. Press, ACS Symp. Ser. 685,225, 1998, Benoit, et al, inter alia, describe use of a nitroxyl radical derived from an alkoxyamine of the type comprehended by the present invention to polymerize n-butyl acrylate. The molecular weights reported at complete conversion are too high for high solids coatings applications and no report of the solids content obtainable is given. This paper, therefore gives no hint that this type of material can be used in the processes of the present invention to provide the results discovered by the inventors.
The invention provides in a first process aspect, a process for the free radical solution polymerization to a level of solids suitable for use as low volatile organic solvent coating compositions while maintaining viscosity suitable for such coating compositions of monomers comprising substituted or unsubstituted acrylic acid, or esters thereof in a solvent suitable for high solids coating application at a monomer concentration sufficient to give the desired polymer concentration which comprises treating said monomers in said solvent with a controlled free radical initiator selected from the group N,N-dialkyl-alkoxyamines having one hydrogen atom on one carbon atom in the a position of one alkyl group attached to the amino nitrogen, N,N-dialkyl-nitroxyl radicals having one hydrogen atom on one carbon atom in the a position of one alkyl group attached to the nitroxyl nitrogen and mixtures thereof.
The tangible embodiments produced by the first process aspect of the invention are solutions containing acrylic polymers having number average molecular weights of less than 20,000, preferably less than 6000 as determined by standard methods well known in the art as set forth hereinafter and a narrow molecular weight distribution, or polydispersity, of less than 2, preferably less than 1.5, as determined by standard methods well known in the art as set forth hereinafter. The reduced molecular weight and narrow polydispersity provide solutions of reduced viscosity allowing for higher solids content at a desired viscosity, thus, providing coating solutions with reduced organic solvent emissions per unit area of surface coated. The lowered viscosity also allows use of polymers with higher molecular weights, if desired, which provides improved physical properties such as hardness and mar resistance. Narrow polydispersity results in fewer monofunctional and nonfunctional chains which also leads to better physical properties. Residual nitroxyl groups attached to the polymer chain are expected to provide light stabilization, thus, providing improved light, particularly ultraviolet (UV) light, resistance and improved weathering properties.
The tangible embodiments are, thus, solutions of acrylic polymers having the inherent applied use characteristics of being suitable for use as lacquers or vehicles for paints for coating applications which have reduced emissions of VOCs while maintaining or exceeding the performance of presently existing analogous lacquers or paints.
The invention also provides in a first composition aspect, a low VOC acrylic based coating composition prepared by the first process aspect of the invention.
The invention further provides in a second process aspect, a process of forming a coating derived from the first composition aspect of the invention to the surface of an object on which said coating is desired which comprises applying a coating composition of the first composition aspect of the invention to said surface and evaporating the solvent from said coating composition.
The invention further provides in a second composition aspect, the coating prepared by the second process aspect of the invention.
The invention still further provides in a third composition aspect, an object having on at least one surface thereof a coating as defined in the second composition aspect of the invention.
One of skill in the art will recognize that the invention also provides as a possible first alternative process variation of the first process aspect of the invention, a process otherwise performed according to the first process aspect of the invention, but with the alternative that the polymer so produced is isolated from the polymer solution produced by the first process aspect of the invention and said isolated polymer may then be further processed by standard methods to, for example, prepare compositions suitable for use in preparing powder coatings.
Thus, the invention provides in a fourth composition aspect a powder coating composition prepared by the above alternative process aspect of the first composition aspect of the invention.
Still further, the invention provides in a fifth composition aspect an object coated on at least one surface thereof with a coating prepared by coating the said at least one surface of the object with the powder coating composition of the fourth composition aspect of the invention and fusing said powder coating composition into a coherent coating.